In order to maximize the hold of a rock bolt or the like in a blind bore, it is known to widen the blind inner end of such a bore with a special tool after it has been drilled out by the conventional bit that leaves behind a bore having a cylindrical wall. Such a bit basically comprises a shaft having a tip from which an undercutting blade projects laterally.
In East German Pat. No. 236,487 the tip end of the shaft is bifurcated and traversed by a pivot pin. The undercutting blade is pivotal on this pin between a position projecting laterally from the tip of the bit and a position recessed within the tip and wholly within the outlines thereof. A ball or other pusher body is dropped into the blind bore after it is drilled but before it is undercut, and the pivotal blade has a formation that engages this ball and pushes the blade laterally outward so that it can undercut the bore when the bit is rotated. Thus as the bit is pushed axially of the bore and longitudinally of the bit into the hole the blade is cammed laterally out so that rotation of the bit undercuts the bore adjacent its inner end.
The main problem with such a tool is that the undercutting blade is fairly weak. Thus the danger of breaking it is fairly great, in particular as such a bit is typically used in rock which is a substance whose hardness is considerable and varying. Furthermore this multipart structure is fairly expensive to produce, so that a bit provided with such moving parts is fairly expensive.